The present invention is directed to a method for processing radio-frequency (RF) signals for Signals Intelligence (SIGINT) applications, and more particularly to an Analog-to-Digital Converter (ADC) analog processing block for pre-processing a signal prior to insertion of the signal to an ADC.
Signals intelligence, commonly referred to as SIGINT, involves searching a large bandwidth of RF spectrum for signals of interest (SOI). Traditionally, SIGINT methods require very high-speed ADCs and extensive processing resources. As more of the RF spectrum becomes allocated for communications, and as communications signals cover wider frequency bands, there are increasing performance requirements placed on the ADC and the associated backend processing system.
For SIGINT applications that cover a wide bandwidth, the sampling rate and the resolution must increase in order to maintain the same overall system performance. The number of interfering signals increases as the amount of RF spectrum that is captured by the ADC increases. The strongest signal detected by the ADC gets mapped by the ADC to a constant voltage, so weaker signals decrease in voltage as the interfering signals get stronger. Hence more processing bits are required in order to resolve the weaker signals of interest.
The primary factors that appear to be limiting ADC performance are aperture jitter and comparator ambiguity. Aperture jitter is created within the sample-and-hold circuit in an ADC. Aperture jitter is related to the random variation in the sampling interval caused by noisy clocks, temperature drift and other instabilities. So to have both high resolution and high bandwidth requires significant improvements in timing. Comparator ambiguity results from the necessity of having each comparator determine, after the hold-time period, whether the result is a 0 or a 1. There is a certain probability that at least one comparator will be indecisive. Aperture jitter and comparator ambiguity have contributed to a decrease in the rate of performance improvement of ADCs, and indicate a trend of fewer SIGINT system improvements that may be achieved by advances in ADC.
Another impediment to SIGINT and ADC performance improvement is the backend SIGINT processing system. The processing performance of the backend SIGINT processing system is driven by the performance of the SIGINT system central processing unit (CPU). Generally, performance improvements have been following Moore's Law, which predicts that CPU performance will double approximately every 18 months. While this theory has held true in the past, recent studies have shown a downward trend in CPU performance advances as well. For example, a study by Ekman, Warg and Nilsson, “An In-Depth Look at Computer Performance Growth”, shows that performance grew by 58% per year between 1985 and 1996. However, during the seven years of 1996-2004, performance increases have slowed to 41% per year. These slowing performance numbers are attributed to three effects:                Total dissipated power        Wire delay        Memory bottlenecks        
The increase in total dissipated power arises primarily from leakage current caused by the smaller line widths needed to increase performance. Wire delay also stems from smaller feature sizes as wire diameters become smaller, as the wire length remains the same or increases. Memory bottlenecks result from larger memory devices that do not match the speeds of the newer CPUs with which the memories are associated. DRAM speeds only increase about 10% per year compared with much larger increases in the operating speed of CPUs.
In order to overcome the limitations of the prior ADC solutions, it is necessary for sampling rates to increase. A higher sampling rate allows a SIGINT system to process RF signals over a wider range of frequencies, as well as to process RF signals that have a very wide bandwidth. Ultra Wide-Band (UWB) systems, for example, already use 500 MHz of RF bandwidth. CPU performance must also increase correspondingly with the wider frequency ranges, in order to process the number of samples that increase proportionately with the frequency range.
Algorithmic performance improvements occur occasionally, however, the advances achieved by algorithmic performance improvements are generally unpredictable, and historically have evolved very slowly, relative to technological improvements in hardware. Often when the backend SIGINT processor fails to keep pace with the signal sampling rate, the default solution is to jettison the excess sampled signals. The SIGINT processor then processes only the most likely signals, or even randomly chosen signals.
Therefore, the entire modem digital SIGINT approach—that of digitizing a broad bandwidth and digitally processing every “signal’within the digitized bandwidth—results in RF signal processing inefficiencies. Some of these inefficiencies may be eliminated by other processing characteristics which are achievable due to the unique processing characteristics of SIGINT.
If a narrow band SOI can be detected reliably only above a certain signal to noise ratio (SNR), then only frequency bins of a certain minimum power need to be inspected. In some applications, SIGINT does not require processing RF signals with a great deal of fidelity. For example, a direct sequence spread spectrum signal detector might be able to use an ADC having only one bit, if the RF signals are properly scaled. Conventional architectures, however, do not benefit by this approach.
Sometimes a particular SIGINT problem only requires a determination that, for example, a certain signal exists, and has a particular modulation and baud rate. It may be unnecessary for the information that is contained within the signal to be demodulated and decoded. It may, for example, be sufficient to determine that the signal existed at a certain frequency at a certain time and, perhaps, at a certain location if the signal can be geo-located. In such a case, very low SNR signals would be candidates for SIGINT processing. The basic SIGINT algorithm could be revised in this case. The SIGINT sampling rate would only need to be sufficiently high to estimate the baud rate and modulation type, rather than meeting a more stringent sampling requirement that might apply if the SIGINT system were designed to demodulate and decode the RF signal itself. But this solution does not meaningfully affect the sampling rate parameters since, according to the Nyquist Theorem, a certain minimum signal sampling rate must be applied to properly sample the entire signal bandwidth.
Another type of SOI that hops in frequency and/or time may be detected simply by the duration of one of its hops. In this case, a first pass algorithm could just sample each frequency bin at the potential hop duration rate looking for one isolated sample with energy above threshold. The processing could adapt to a changing interference signal environment, thereby reducing the overall system requirements. For example, if the environment was free of interfering signals, the presence of a SOI could be determined using filters with wider RF bands, that require fewer taps, and hence less processing. This technique requires the SIGINT system to adapt in both frequency and time to constantly changing conditions.
For direct sequence, spread spectrum (DS-SS) signals, the only reliable means of detection is to have advance knowledge of the DS-SS signal sequence, and to use correlation to raise the DS-SS signal above the background noise. This type of detection means imposes an enormous burden on the backend SIGINT processor.
The various aforementioned techniques may, in some cases, have alternate means of implementation that do not require digitizing a very wide bandwidth and then processing every RF signal using a digital algorithm. However, implementation of these techniques requires the entire SIGINT architecture to be redesigned.
Currently, there is no practical means available for searching the entire 10 GHz of RF spectrum that is commonly used for communications. Most SIGINT systems are more specialized or are restricted to searching much narrower bandwidths. Prior solutions to the general SIGINT problem required sampling and processing of the entire RF spectrum by the SIGINT system. Sampling and processing the entire RF spectrum imposes a severe burden on the ADC and associated backend processor performance. Alternate methods which might use fewer samples and less processing resources are often times not possible to implement in prior solution architectures.
Therefore what is needed is a SIGINT system that does not require sampling and processing the entire RF spectrum and that may be implemented in existing SIGINT architectures.